Bulbasaur - Comes at Level 12 via in-game event, seems alright in terms of usual Bulbasaur shenanigans. It isn't very far off from learning Poison Powder / Sleep Powder and Vine Whip is still an okay STAB for a while. Mega Drain, I assume, is a TM via Eirika which would help push for it. Seems solid overall as always, nothing really to complain. If you can get it as soon as Curelean City it'll be a solid addition.
Charmander - Seems "okay", at least can help against Eirika when trained, but it learns a lot of pivotal moves very late. Dragon Rage, for example, could have been a really nice move since a lot of Rock-types have pretty low HP. It doesn't help that the move it learns next for a Fire-type attack is Fire Spin, at Level 33. If I had to be honest I don't think it will be very good, but it hinges on when the Flamethrower / Fire Blast TMs are available for it.
Squirtle - Joins at a rather awkward time, I believe right before Lt Surge or right after, and at least can have a footing in Rock Tunnel vs some trainers. Bubble Beam is a bit later than I like it, but Surf or Scald TMs should help it out a bit if they're available at reasonable times. I feel Squirtle will be more middle-pack this time around.
Caterpie - STAB Gust at Level 10 helps, and it gets some very good moves pretty early on. Having a complete moveset with Quiver Dance / Air Slash / Bug Buzz / Psybeam (Psychic with TM) isn't too shabby. There's also Mega Drain TM which helps. Only problem is you probably shouldn't really use this mon for a super long time, but at least it seems more optimal to use early on in the game.
Weedle - Level 10 Twineedle is pretty nice to have, and Poison Jab at Level 22 is respectable. Pin Missile isn't even very far at Level 26. Problem is sort of the same with Caterpie - probably shouldn't really use it very long, but likely serviceable. If it can get the Mega early on it would definitely push its viability up a lot.
Pidgey - About the same-ish as usual. The major change is it might be wise to hold back on the evolution to get Air Slash at Level 39 (Pidgeotto). Still, unless it gets its Mega it's not really a great Flying-type and is inferior to Dodrio later on and Spearow earlier.
Rattata - This things movepool got changed dramatically. Super Fang now comes before Hyper Fang - at Level 15. You might want to hold off on evolving it a level so it gets Hyper Fang at Level 21. Since it has solid Dark-type moves in Crunch and Sucker Punch its worthwhile to keep around for a short time, but long-term you're definitely going to want something else. Alolan Raticate does outclass vanilla Raticate, though.
Spearow - Kind of hinges on a few conditionals - 1) where Fly is and 2) where move relearner is and how hard it is to relearn moves. Drill Run is nice and it's also a TM, so that could also help. Still wouldn't pit it against most Rock-types, but it's an option for desperate scenarios that should be taken into account.
Ekans - Poison Jab comes at Level 24 as Ekans, so you'll probably want that. Has Glare, which is nice at least, but nothing special. Crunch is an evolved move, so there's small utility against Gastly lineup.
Pikachu - Double Kick is now learned at Level 9, but with the Let's Go Pikachu you'll likely never touch regular Pikachu. I feel without Alolan Raichu Pikachu (vanilla) is only good for earlygame. It learns Thunderbolt at Level 21 which is really nice though.
Sandshrew - Finally this fucker learns Dig via Level up. This actually pushes it to a pretty viable Ground-type to use, though questionable if Dugtrio still outmatches it. It does learn Swords Dance naturally, so there is a small niche to using Sandslash, but you'll see later that Dugtrio has a nasty ace.
Nidorans - I'll just group both because they're about the same. Male is definitely superior in terms of levelup movepool, and Female is still "okay" but nothing to write home about. They ride or die via TMs, which is normal for them.
Clefairy - Versatile, but that's about it. Fairy-typing is kind of a mixed blessing and it depends on if we see any Alolan Pokemon join the fray as opposing Pokemon. Otherwise, being weak to Poison-types is kinda awful in that regard. Moonblast is kind of late at Level 28, which is "okay" but being only encounterable at Level 5-10 is really awful and shows how much time it'll take. It has a wide TM movepool, though, so it does salvage itself pretty well in that regard. If Dazzling Gleam is early enough it should be alright, though.
Vuplix - Level 24 Flamethrower is really damn good. Sucks that it is pretty linear with only Flamethrower (and Dark Pulse / Solarbeam) for moves, but hey it's workable and that's what counts.
Jigglypuff - Much like I said above with Clefairy kind of applies with Jigglypuff. Small differences include that it has Normal-type STABs, which does help it a little bit, but a lot of them aren't very good (Body Slam is kinda late at 22, Doubleslap is not a good move). If Headbutt is early enough things should be okay for it. It's not really that strong as Wigglytuff and while its versatile like Clefable its stats are a bit worse. Lacking Hyper Voice kind of sucks too. There is Tri Attack though...
Zubat - Though no Crobat, Zubat line isn't too shabby. It learns real moves early on (Absorb is actually an okay move now, for starters) and Wing Attack is nice and early. Scales kind of meh lategame, but that's really the only problem with it. Still works fine for a while, and a worthwhile addition early on I'd say.
Oddish - Honestly, a good lineup. Level 22 Mega Drain (hold a level before evolving) is solid, and STAB Acid and Absorb earlygame is really nice. Catch this one at Level 12 at the latest or you'll be without a Grass-type STAB until Level 18 (Razor Leaf). Move tutor gives Solarbeam for Vileplume.
Paras - Okay so it learns Sleep Powder early which actually is nice. Growth early on isn't too shabby, and Leech Life comes at Level 23 to help it out a bit. It lacks a strong Grass-type move, though, and relies on Mega Drain via level-up or Solarbeam off of an eh 60 SpA stat. Nothing really wonderful I'd say but can still work okay early in the game.
Venonat - So Venonat now comes pretty early in Route 24. No real STABs early on and Confusion / Psybeam (Level 20) is okay though. Leech Life is kind of late at Level 28 though, but Psychic at 32 (hold off on evolving a level) isn't too bad. Venomoth gets Bug Buzz and Quiver Dance at Levels 46, and 55, respectively, which sounds late.
Diglett - Earthquake at Level 29 (hold off on its evolution) is solid, and Dig at Level 18 (so catch the Level 18 Diglett) is pretty good. The only downside is 0 durability whatsoever and, if the opponent gets some durability boosts too, it might be troublesome for Dugtrio to snag KOes before it's KOed. Hinges a lot on that factor, but sounds very good otherwise.
Meowth - Solid starting moves, but sucks that it lacks Technician to make use of it. Level 14 Meowth can have Fake Out / Bite / Taunt / Pay Day. Slash at Level 27 isn't too bad either. Gets Nasty Plot and Swift, but Nasty Plot is really late (Level 37 as Persian, 31 as Meowth so 3 levels held off from evolving), but 70 Atk / 65 SpA isn't really doing favors this time around.
Psyduck - Available Route 25, Psyduck has a tolerable movepool with Water Gun and Confusion early on. Nice to get Psybeam early at Level 27 which means it can hold off on Psychic from TM, and if you hold back a few levels Surf is Level 36. 82 base Attack isn't too bad to use stuff like Ice Punch (if it's available). Usable.
Mankey - Catching the Level 12 Mankey means the next level it will learn Karate Chop (oddly it doesn't have Low Kick until Level 21). U-turn at Level 29 (hold Mankey a level) isn't too shabby, and if the elemental punch TMs are available early enough on it's not too shabby.
Growlithe - Ho boy, this thing looks so good. LEVEL 17 FLAMETHROWER. Do I really need to say anything else? Heat Wave at Level 24 isn't bad either if for some reason you encounter double battles. This thing actually can be pretty damn strong if it evolves almost right away, but it doesn't hurt to keep it at Growlithe since Flare Blitz isn't exactly "late" (38) and base 70 offensive stats prior to are usually "okay".
Poliwag - This thing got really solid tbh. Low Kick is at Level 26 (hold off on evolving Poliwag one level) and you still have Bubble Beam for a solid STAB. The ONLY problem is this is encountered via Water. What do we have to do with that, you may ask? I don't know dear sir, but that sounds pretty late. Still, Poliwrath looks pretty damn solid with a well-rounded movepool and good TM options. Small note - Submission is learned via evolution to Poliwrath too.
Abra - We'll have to see how the leveling curve works, but Abra line seems to be working out swell. Psybeam at Kadabra is going to clock the ever living fuck out of mons. Supposedly it also learns Calm Mind via evolution to Alakazam - a nice saving on time for TMs. Aboslutely busted IMO - especially if there's a severe lack of Dark-types.
Machop - Brick Break / Karate Chop / Seismic Toss / Bide is likely its starting moves. Submission is a pretty easy pass with Brick Break being this early on, and Bulk Up is available if you hold Machop to Level 31. Strength is available for free as Machamp which at least gives it another move to use, but nothing that screams "I need it". The real question is where tf Cross Chop went for Primeape and Machamp...
There are elemental punches too.
Bellsprout - Level 22 Bellsprout (so hold off on a level for evolution) learns Razor Leaf. If you can stomach another 4 levels (eh) you get Poison Jab too. Victreebel has a lot of moves it learns via Move Tutor - Power Whip, Leech Life, Swords Dance, Sucker Punch, and also Clear Smog (uh... okay). I'd say pretty solid overall since it has the right moves to function physically and mildly on the special side too (Mega Drain / Sludge Bomb via TMs).
Tentacool - All Water encounters. Seafoam is Level 39-44, so not sure if it's locked from everything or something? Anyway, that means it'll have Surf and can have Hydro Pump at Level 45 (Surf is Level 27). Bubble Beam at Level 18 isn't too shabby. Tentacruel kind of relies on TMs to keep itself going, but having STAB on Sludge Bomb is nice when you teach it to Tentacruel.
Geodude - Rock Throw at Level 10 is workable, and Level 23 Rock Slide is nice. So is Level 27 Earthquake if you hold it two levels. Elemental punches makes this thing potentially cool too. No Rock Polish so prepare to be slow.
Ponyta - Honestly, it looks so bad. Double Kick is its only non-Normal- and Fire-type move outside of Megahorn (laughable), Iron Tail (...), and Solarbeam (yay 2 turns off of inferior SpA). Oh Poison Jab too. Just... don't bother. Level 65 Flare Blitz alone should speak volumes against using it.
Slowpoke - Actually because it comes in at Level 39-44 it matters a lot. This means that at Level 42 Slowpoke will have both Surf and Psychic. Slowbro has a pretty good movepool with Flamethrower as an extra move it can have in its arsenal on top of Ice Beam. Really late at Seafoam Islands though.
Magnemite - These come in at pretty high levels too (37-42 at the Power Plant). This means it'll have Thunderbolt and Flash Cannon for STABs, which is all it really needs. Magneton naturally learns Tri Attack at evolution, so while not a great option it's an okay option to have. Just joins super late and at a questionable time too since by the time this is available you're likely not using it versus Blaine nor Giovanni.
Farfetch'd - Rather late encounter for it (Route 12, Level 31-36). Level 36 has Swords Dance but no hold items kind of ruins its only niche.
Doduo - Oh boy, so you can catch one on Route 17 or Route 18 at Level 38. That's dangerous because it has access to Drill Peck, Swords Dance, AND Jump Kick. Only thing that sucks is Thrash is super late and all you have otherwise is Facade / Headbutt, but that's more a nitpick than a con. Hard addition IMO.
Seel - Catching Level 44 Seel gives Ice Beam and Waterfall for STABs. Dewgong's offenses are kind of meh, though. At least Water / Ice STABs are really good.
Grimer - It's late, it learns the wrong moves... the way they made Grimer and Muk is so bad in-game it hurts. Poison Jab being a TM is at least a saving grace maybe, and elemental punches / Rock Slide too. It's super late at Pokemon Mansion, though, and only holds very small niches against some things. Also, why in the ever living fuck did they give it Moonblast?
Shellder - Seafoam Islands encounter, and you'll want to find the one at Level 44. Though late, you can have a Level 45 Cloyster with Shell Smash / Ice Beam / Hydro Pump / Spike Cannon which is pretty damn dangerous.
Gastly - It automatically has Shadow Ball since encounter is 27-32. That's... pretty damn good. Needs TMs but very workable.
Onix - Look, it has 45 Attack. I'm not even going to bother this one. It can be found earlier (at a laughable 1% in Mt Moon), and I guess if you train it 10 levels it learns Dig. But, why...
Drowzee - Okay level up movepool with Psybeam at Level 22, but that's about it. Elemental Punches via TM is nice, but 73 offenses are pretty meediocre.
Krabby - Says it can be caught on Route 10... at Level 18-23. The problem? Crabhammer is Level 36... as Krabby. If you can hold off to Route 13 you can catch a Krabby up to level 38. Kingler shockingly lacks Swords Dance but has Agility, which is kind of odd. Really dependant on TMs for coverage.
Voltorb - Super late at Power Plant... and that's about it. Has Thunderbolt... I guess...
Exeggcute - Very late at Victory Road, Exeggcute comes at a pretty high level at least (41-46). This at least has it have Reflect / Leech Seed / Psybeam / Sleep Powder. You can either level to 48 for SolarBeam or evolve and tutor it for Power Whip. Being really damn late hurts it a lot but it has a couple niches maybe for the E4.
Cubone - Level 32 Cubone can have Thrash / Bonemarang which is pretty nice, which is as early as... Pokemon Tower. Yeah, the leveling scaling is weird. Gets Swords Dance as Marowak which, thank God, because if it didn't this thing would be absolutely fucked without Thick Club.
Hitmonlee - A Level 30 gift, and clearly superior. Jump Kick / Facede / Brick Break / Feint should be the starting moves. Learns HJK at 45 which is okay. Rock Slide is all it really needs for coverage, and maybe Earthquake if Nidos are common fsr.
Hitmonchan - El. Oh. El. Dizzy Punch / Thunder Punch / Ice Punch / Fire Punch are its starting mvoes. It doesn't learn a Fighting move barring Counter... yeah. Brick Break and Superpower are needed for TMs to even make this thing worth LOOKING at.
Lickitung - It's available in Curelean Cave and it's awful. 55 base Attack is deplorable. Do not touch.
Koffing - Sludge Bomb can come from the later Koffing (Level 42). It's lacking a lot otherwise outside of TMs but it definitely can be workable.
Rhyhorn - Okay so fun fact - this thing comes as early as Rock Tunnel at Level 18-23. That means Rock Throw naturally and Drill Run at Level 24. The late evolution and it being super slow / lacking in Special bulk still hurt it tremendously, but this things movepool gets scary as the game progresses. Picture - Rock Slide at Level 43, Megahorn at Level 36, and you still have access to TMs. Were not for the next gym leader it might face be Koga at best, this thing got massively buffed regardless.
Chansey - Curelean Cave only. I mean look it's Chansey. 35 SpA and 5 Atk isn't doing anything. You desperately need Seismic Toss and your move tutor to re-learn Soft-Boiled.
Tangela - Eh. Late enough level makes it okay. Mega Drain is workable. You don't want Power Whip off of 55 base Attack. Should do okay vs Giovanni I guess?
Kangaskhan - Now found in Rock Tunnel (at 1%), It's severely lacking in STABs unless you consider Fake Out one of them. If you can hold off to Level 28 it gets better with Mega Punch. So long as Headbutt TM is early might salvage it. It's definitely okay for a Pokemon but it's got flaws since it's a Normal-type heavily reliant on TMs.
Horsea - Seadra is honestly workable but just barely at base 95 SpA. I wouldn't really get too excited to use it. It learns Dragon Pulse naturally but... meh.
Goldeen - It's a Water Encounter which alone makes things very questionable. If it can be caught that early you could actually make use of it, believe it or not. Waterfall comes naturally at Level 24 which isn't that bad. Otherwise, though, this thing is pretty bad.
Staryu - Can be found in Route 21 or Seafoam Islands around Level 37-44 ranges. Might be worthwhile to wait for Seafoam because Hydro Pump is Level 45 and you still have Psychic and Recover. Still, Surf / Scald works otherwise and there's Thunderbolt / Ice Beam as potential TMs.
Mr. Mime - So Route 11 can have Mr Mime. It's... okay. The problem is there's no real good STABs for it until Level 30, or Dazzling Gleam / Psychic via TMs.
Scyther - Route 14 and 15 have Scyther now.The biggest problem is that you want to avoid Level 38 since it replaces Wing Attack with Air Slash. Kind of hinges on TMs IMO since X-Scissor is Level 50 and Swords Dance is Level 44. Brick Break is there too, so it's okay but nothing outstanding.
Jynx - Seafoam Islands. Basically should have its STABs + Lovely Kiss by Level 45, though you probably want Ice Beam > Blizzard via TM.
Electabuzz - Though it's 2 levels off Thunderbolt is solid (44). Versatile movepool helps this thing a lot too.
Magmar - See above with Electabuzz, replace Thunderbolt with Flamethrower, and you get the same thing with Magmar. Problem is unlike Electabuzz it struggles in the later stages when you consider the gym leaders it faces are Fire, Ground and then the E4 has Water-types. Also don't forget this thing doesn't learn Thunderbolt, only Thunder Punch.
Pinsir - It'll have its STAB and Swords Dance. It's workable, but nothing outstanding. Kind of like Scyther in that it's pretty meh.
Tauros - Route 14 and such. Level 38 is the max, which means Focus Energy / Take Down / Rest / Leer (Horn Attack if you get Level 37). Thrash at 42 helps a bit, but like Kangaskhan it relies a bit on Tms and is arguably a little worse than Kangaskhan since Kanga can get elemental punches.
Magikarp - Yeah we'll have to see how leveling works. Gyarados seems fine otherwise - Waterfall at Level 31 makes it serviceable. No Intimidate makes it a bit frailer though.
Lapras - Free at Silph Co at Level 35, which means it's closeish to Ice Beam at Level 39. TM for Surf should help round it out nicely. Thunderbolt access and Psychic is nice too.
Ditto - L.
Eevee - Excluding Let's Go Eevee (which I'll say is pretty solid), the Eeveelutions definitely are polarizing. On one hand your gift Eevee is Level 30, which means it misses out on a lot of potential moves. When it evolves, it will learn Water Gun (Vaporeon), Ember (Flareon), and Thundershock (Jolteon). The awkward thing is that at Level 31 they learn Hydro Pump, Flare Blitz, and Thunder... so what's the point of it being that late? Furthermore Aurora Beam is Level 17 for Vaporeon. I would say in viability Vaporeon > Flareon > Jolteon. Something I thought I'd never say, but let's be honest with Jolteon for a moment that the only other special move it has is Shadow Ball which doesn't help it a lot. Flareon could still lose depending on how late the TMs come - that's the kicker. Superpower is really important for it, and being slow + taking a lot of recoil does suck for it too.
Porygon - Like... bleh. It's a gift at Silph Co too. Really slow. Guess Tri Attack helps a bit.
Omanyte - Depends on fossil level.
Kabuto - Depends on fossil level.
Aerodactyl - Depends on fossil level.
Snorlax - STAB Headbutt early on works, but Body Slam is incredibly late. In fact, so is Crunch and Superpower. TMs can salvage this though. Just sucks that there isn't really a good Normal-type TM otherwise.
Articuno - Static encounter. Ice Beam is nice.
Zapdos - Static encounter. Drill Peck and Thunderbolt are nice.
Moltres - Static encounter. Flamethrower is nice. Why did this thing lose Sky Attack but Zapdos gets it like ??? it's minor but still.
Dratini - Really hinges on when Dragon Pulse is available IMO, and even then, the late evolution kind of reeks.